1. Field of the Invention
This invention relates to a method for producing aliphatic O-arylurethanes widely used as masked isocyanates, or as precursors of aliphatic isocyanates. Particularly, this invention relates to a method for producing aliphatic O-arylurethanes characterized in that an aliphatic primary amine is reacted with an aromatic hydroxyl compound and urea while the ammonia produced as a by-product is removed from the reaction system so that the ammonia concentration of the reaction solution is maintained at 2 wt.% or less.
2. Discussion on Related Art
Hitherto, aliphatic O-arylurethanes have been produced by reacting an aromatic hydroxyl compound with an aliphatic isocyanate (for example, K. Iwata, Lecture on Plastic Materials 2, Polyurethane Resins, pp. 174.about.178, published by Nikkan Kogyo Shinbun-sha (Daily Industries News Co., Ltd.), 1969; K. C. Frisch, "Fundamental Chemistry and Catalysis of Polyurethanes", Polyurethane Technology, P. F. Bruins, Ed., Interscience Publishers, New York, 1969, pp. 11.about.12). The aliphatic isocyanate is obtained by reaction of the corresponding aliphatic primary amine with phosgene (see for example, Brit. Patent No. 1,077,031), so that this method has the following drawbacks: (1) phosgene, a deadly poison is used; (2) corrosive hydrogen chloride gas is produced as a by-product in large amounts; and (3) the product usually contains hydrolyzable chlorine compounds, and these by-products are extremely difficult to remove. Consequently, the method of obtaining aliphatic O-arylurethanes by the reaction of an aromatic hydroxyl compound with an aliphatic isocyanate has not been satisfactory.
U.S. Pat. No. 4,297,501 discloses a method for producing aliphatic O-arylurethanes without using phosgene by oxidative urethanation of a primary amine with carbon monoxide and an aliphatic alcohol or aromatic hydroxyl compound in the presence of a noble metal catalyst. This patent, however, discloses no examples in which the aromatic hydroxyl compound is used. This method also has several drawbacks: (1) poisonous carbon monoxide is used; and (2) since an expensive noble metal catalyst is used, it must be recovered from the resulting urethane products, and this recovery requires troublesome operations and is expensive.
U.S. Pat. No. 3,873,553 discloses a method for producing N-alkyl-O-arylurethanes by reacting N-alkyl-N',N'-dialkylurea with an aromatic hydroxyl compound and hydrogen chloride gas. This method also has drawbacks that highly corrosive hydrogen chloride gas and an expensive and special urea compound are used, and that recovery of the urethanes from N,N-dialkylamine hydrochloride, a by-product, requires troublesome operations and is expensive.
U.S. Pat. No. 2,677,698 discloses a method for producing aliphatic monourethanes without using phosgene in which N,N'-dialkylurea is produced in a first step by reacting an aliphatic primary monoamine with urea, and in a second step, the aliphatic monourethanes are produced by reacting N,N'-dialkylurea with a hydroxyl compound, and the primary amine, a by-product, is separated, recovered and recycled to the first step. This patent, however, discloses no examples in which the aromatic hydroxyl compound is used. This method is also not satisfactory to carry out industrially because the yield of the urethane is low, and the process is very troublesome, since the reaction is carried out in two steps and equipment for recycling the primary amine is required.
Some methods for producing aliphatic urethanes by reacting an aliphatic primary amine with a hydroxyl compound and urea in one step have been proposed. The aliphatic urethanes obtained by these methods, however, are not aliphatic O-arylurethanes, but aliphatic O-alkylurethanes. For example, U.S. Pat. No. 2,409,712 discloses a method for producing aliphatic O-alkylmonourethanes by reacting an aliphatic primary amine with urea with an aliphatic alcohol. German Patent Nos. 2,917,493 and 2,943,551 also discloses a method for producing aliphatic O-alkylpolyurethanes by reacting an aliphatic primary polyamine with an aliphatic, alicyclic or aralkyl alcohol in the presence of urea or a urea compound.
However, the aliphatic O-alkylurethanes produced by these methods, because they are thermally very stable, are difficult to decompose to the corresponding aliphatic isocyanates and alcohols. They are not, therefore, satisfactory to use as masked isocyanates or as precursors of aliphatic isocyanates.
In this respect, it is known that the aliphatic O-arylurethanes easily decompose to the corresponding aliphatic isocyanates and aromatic hydroxyl compounds (for example, O. Bayer, "Das DiisocyanatPolyadditions Verfahren", p. 12, 1963). However, a method for producing aliphatic O-arylurethanes by reacting an aliphatic primary amine with an aromatic hydroxyl compound and urea in one step was not known.